Switching of perpendicularly magnetized nanomagnets with spin-orbit torques in the absence of external magnetic fields

ABSTRACT

A base element for switching a magnetization state of a nanomagnet includes a heavy-metal strip having a surface. A ferromagnetic nanomagnet is disposed adjacent to the surface. The ferromagnetic nanomagnet has a first magnetization equilibrium state and a second magnetization equilibrium state. The first magnetization equilibrium state or the second magnetization equilibrium state is settable in an absence of an external magnetic field by a flow of electrical charge through the heavy-metal strip. A method for switching a magnetization state of a nanomagnet is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S.provisional patent application Ser. No. 62/158,805, SWITCHING OFPERPENDICULARLY MAGNETIZED NANOMAGNETS WITH SPIN-ORBIT TORQUES IN THEABSENCE OF EXTERNAL MAGNETIC FIELDS, filed May 8, 2015, whichapplication is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

This invention was made with Government Support under Contract GR526115and GR526116 awarded by the Intelligence Advanced Research ProjectsActivity (IARPA). The Government has certain rights in the invention.

FIELD OF THE APPLICATION

The application relates to switching the magnetization of nanomagnetsand particularly to a base element structure and method for switchingthe magnetization of nanomagnets.

BACKGROUND

Complementary metal-oxide-semiconductor (CMOS) technologies areprevalent today in memory and logic systems. However, CMOS technologiesno longer provide a desired balance of fast operation, high densityintegration, and energy efficiency.

SUMMARY

According to one aspect, a base element for switching a magnetizationstate of a nanomagnet includes a heavy-metal strip having a surface. Aferromagnetic nanomagnet is disposed adjacent to the surface. Theferromagnetic nanomagnet has a first magnetization equilibrium state anda second magnetization equilibrium state. The first magnetizationequilibrium state or the second magnetization equilibrium state issettable in an absence of an external magnetic field by a flow ofelectrical charge through the heavy-metal strip.

In another embodiment, the flow of electrical charge in a firstdirection through the heavy-metal strip causes the first magnetizationequilibrium state and the flow of electrical charge in a seconddirection through the heavy-metal strip causes the second magnetizationequilibrium state.

In yet another embodiment, the nanomagnet includes an elliptical shapehaving a long axis and a short axis.

In yet another embodiment, the long axis is about parallel to thesurface of the heavy-metal strip.

In yet another embodiment, a direction of flow of the electrical chargethrough the heavy-metal strip includes an angle ξ with respect to theshort axis of the nanomagnet.

In yet another embodiment, the angle ξ determines an energy ofswitching.

In yet another embodiment, the angle ξ determines a speed of switching.

In yet another embodiment, the base element provides a bit of anintegrated memory device.

In yet another embodiment, the base element provides a bit of anintegrated logic device.

In yet another embodiment, the base element provides a bit of anintegrated pipelined microprocessor device.

In yet another embodiment, the heavy-metal strip includes tungsten ortantalum.

In yet another embodiment, the heavy-metal strip includes Aluminum (Al)or Gold (Au).

In yet another embodiment, the heavy-metal strip includes Bismuth (Bi)or Molybdenum (Mo).

In yet another embodiment, the heavy-metal strip includes Niobium (Nb)or Palladium (Pd).

In yet another embodiment, the heavy-metal strip includes Platinum (Pt).

In yet another embodiment, the heavy-metal strip includes an alloy ofcopper (Cu) and Bi, or an alloy of Cu and iridium (Ir).

According to another aspect, a method for switching a magnetizationstate of a nanomagnet includes the steps of: providing a heavy-metalstrip having a surface and a ferromagnetic nanomagnet disposed adjacentto the surface, the ferromagnetic nanomagnet having a firstmagnetization equilibrium state and a second magnetization equilibriumstate; flowing an electrical charge through the heavy-metal strip in anelectrical charge direction to set the magnetization state of thenanomagnet in an absence of an external magnetic field to the firstmagnetization equilibrium state, or to set the magnetization state tothe second magnetization equilibrium state.

In another embodiment, the step of flowing an electrical charge includesflowing an electrical charge through the heavy-metal strip in anelectrical charge direction to set the magnetization state within a timeperiod of less than about 50 picoseconds.

In yet another embodiment, the step of flowing an electrical chargeincludes flowing an electrical charge through the heavy-metal strip inan electrical charge direction to set the magnetization state where themagnetization state corresponds to setting a bit of a memory device.

In yet another embodiment, the step of flowing an electrical chargeincludes flowing an electrical charge through the heavy-metal strip inan electrical charge direction to set the magnetization state where themagnetization state corresponds to setting a bit of a logic device.

The foregoing and other aspects, features, and advantages of theapplication will become more apparent from the following description andfrom the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the application can be better understood with referenceto the drawings described below, and the claims. The drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles described herein. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a drawing showing a ferromagnetic layer adjacent to aheavy-metal nonmagnetic nanostrip;

FIG. 2 is a drawing showing how the dynamics of the magnetization motioncan be captured by ϑ and φ;

FIG. 3A is a drawing showing that the charge current (J_(e)) injectedthrough the nonmagnetic heavy-metal induces spin current (J_(s));

FIG. 3B is a drawing showing an exemplary elliptical ferromagnet havingan in-plane anisotropy H_(kx);

FIG. 3C is a drawing that shows the charge current direction andorientation of the spin polarization a with respect to H_(kx);

FIG. 4 is a drawing showing the motion of magnetization under theinfluence of spin-torques and anisotropies; and

FIG. 5 is a drawing showing the trajectory of the magnetizationswitching of a ferromagnetic layer using spin-orbit torques in theabsence of an external magnetic field.

DETAILED DESCRIPTION

Magnetization switching of ferromagnets using spin-orbit torquesprovides opportunities to introduce nanomagnets into high performancelogic and memory applications requiring low power consumption.Nanomagnets with perpendicular-to-the-plane anisotropy have recentlyattracted a considerable attention due to their high thermal stability.High stability against thermal fluctuations allows nanomagnets to bedeeply scaled down, resulting in dense logic and memory systems withultra-low power consumption. However, due to the symmetric energylandscape experienced by the magnetization of a nanomagnet withperpendicular-to-the-plane anisotropy, spin-orbit torques induced by anin-plane current pulse cannot switch the magnetization. An externalmagnetic field is, therefore, required to assist spin-orbit torques bybreaking the symmetry. Although the energy dissipated by switching ananomagnet could be small, the energy necessary to generate the requiredmagnetic field makes the overall memory or logic scheme uncompetitive ascompared to complementary metal-oxide-semiconductor (CMOS) counterparts.Additional metals are also necessary to produce the required magneticfield, significantly decrease the number of devices which can beintegrated over a given area. Therefore, the need for an externalmagnetic field is an obstacle for developing dense low power memory andlogic systems. Furthermore, fast switching requires higher energy to beinjected through the ferromagnet and/or metals producing magnetic field.Since the required energy grows significantly as the desired switchingspeed increases, fast operation compromises the energy efficiency.

A solution to the problems described hereinabove switches themagnetization of a nanomagnet with perpendicular-to-the-plane anisotropyusing spin-orbit torques induced by an in-plane current pulse withoutthe presence of an external magnetic field.

The solution includes a scheme to switch the magnetization of ananomagnet with perpendicular-to-the-plane anisotropy using spin-orbittorques induced by an in-plane current pulse without the presence of anexternal magnetic field. It was realized that magnetization switchingcan be achieved by breaking the symmetry by introducing an in-planeanisotropy into the nanomagnet. We describe how spin orbit torquesinduced by an in-plane current pulse of appropriate amplitude andduration are sufficient to switch the magnetization of the nanomagnet inabsence of an external magnetic field. For a given ratio between thein-plane and perpendicular-to-the-plane anisotropies, high switchingprobability (deterministic switching) is achievable for current pulsesof significantly short duration by balancing the spin-orbit and dampingtorques, resulting in ultra-fast switching. Furthermore, since externalmagnetic field is not required for magnetization switching within thedescribed scheme, energy efficiency and integration density issignificantly improved, resulting in ultra-fast dense memory and lowpower consumption logic systems.

FIG. 1 shows an exemplary ferromagnetic layer including aperpendicular-to-the-plane anisotropy (H_(kz)) and an in-planeanisotropy (H_(kx)) is situated on a heavy-metal nanostrip. In oneexemplary embodiment, the proposed structure of base element 100, FIG. 1shows a ferromagnetic layer represented by a Stoner-Wohlfarth monodomainmagnetic body 101 with magnetization M, situated at a heavy-metalnonmagnetic nanostrip 102 with strong spin-orbit coupling. Theferromagnetic layer, as shown in FIG. 1, includes aperpendicular-to-the-plane uniaxial anisotropy H_(kz) along the e_(z)axis and an in-plane uniaxial anisotropy H_(kx) along the e_(x) axis.

FIG. 2 shows how the dynamics of the magnetization motion can becaptured by ϑ and φ. As shown in FIG. 2, the motion of M is representedby a unit vector n_(m), which makes an angle ϑ with e_(z) axis, whilethe plane of M and e_(z) makes an angle φ with e_(x).

FIG. 3A shows that the charge current (J_(e)) injected through thenonmagnetic heavy-metal induces spin current (J_(s)). As shown in FIG.3A, a charge current J_(e), injected through the heavy-metal nanostrip,produces a traverse spin current J_(s)=θ_(SH)(σ×J_(e)) due to thespin-orbit interaction, where J_(e) is the charge current density, σ isthe spin polarization unit vector, and θ_(SH) is the material dependentspin Hall angle.

FIG. 3B shows an illustration of an exemplary elliptical ferromagnethaving an in-plane anisotropy H_(kx).

FIG. 3C shows the charge current direction 301 and orientation of thespin polarization σ with respect to the H_(kx). As shown in FIG. 3C, thedirection of the charge current J_(e) makes an angle of ξ with e_(y)axis. Spin polarized current transports spin angular momentum into thenanomagnet, exerting a torque on the magnetization. The dynamics of Munder the influence of torques and anisotropy fields is described usingthe Landau-Lifshitz-Gilbert (LLG) equation as

$\begin{matrix}{{\frac{{dn}_{m}}{dt} = {{- {\gamma \left( {n_{m} \times H_{eff}} \right)}} + {\alpha \left( {n_{m} \times \frac{{dn}_{m}}{dt}} \right)} + {\gamma \; T_{ST}}}},} & (1)\end{matrix}$

where γ is the gyromagnetic ratio, α is the damping factor, T_(ST) isthe spin torque, and H_(eff) is the effective field experienced by themagnetization of the ferromagnetic layer. H_(eff) is a function ofH_(kx) and H_(kZ). The spin torque has two components, referred to asthe in-plane and out-of-plane torques: T_(ST)=T_(IP)+T_(OOP).

FIG. 4 shows the motion of magnetization under the influence ofspin-torques and anisotropies. As demonstrated in FIG. 4, the in-planetorque T_(IP) lies in the plane defined by M and H_(eff) , and theout-of-plane torque T_(OOP) points out of the plane defined by M andH_(eff) .

FIG. 5 shows the trajectory of the magnetization switching of aferromagnetic layer using spin-orbit torques in the absence of anyexternal magnetic field. As shown in FIG. 5, by injecting charge currentJ_(e) through the heavy-metal nonmagnetic nanostrip, produced spintorques derive M out of the equilibrium position (also called anequilibrium state) toward the in-plane of the nanomagnet. By turning thecharge current J _(e) off after t_(e) seconds, spin torque reduces tozero and M is close to the x-y plane and away from the e_(z) axis by anangle of ϑ. At this zone, referred here to as the critical zone, H_(eff)is significantly dominated by H_(kx). Therefore, M passes the hard axisby precessing around the H_(eff) . By passing the hard axis, H_(eff) isdominated by H_(kz). Hence, M is pulled towards the new equilibriumstate by precessing and damping around H_(eff), completing themagnetization switching.

The duration t_(e) of the applied current pulse is as short as the timewhich causes the magnetization M to move from the equilibrium state tothe critical zone. The magnetization switching can be performed usingcurrent pulses of a duration of sub-50 ps. Therefore, the proposedscheme significantly improves the switching speed and/or reduces theenergy consumption, resulting in ultra-high-speed spin-torque memory andlogic systems which have significantly low energy consumption.Furthermore, as no extra metal is required for producing an externalmagnetic field, integration density is considerably enhanced.

It is contemplated that both switching energy and switching speed can bedetermined by the angle ξ. It is also contemplated that there is atradeoff between switching energy and switching speed as can be set bythe angle ξ.

Heavy-metals as used hereinabove include any suitable transition metalshaving a large atomic number, such as, for example, tungsten (W),tantalum (Ta), Aluminum (Al), Gold (Au), Bismuth (Bi), Molybdenum (Mo),Niobium (Nb), Palladium (Pd), or Platinum (Pt). Also included are anysuitable metal alloys, such as, for example, an alloy of copper (Cu) andBi, or an alloy of Cu and iridium (Ir). By injecting a charge currentthrough a heavy-metal thin film of any suitable metal or metal alloy aslisted hereinabove, a traverse spin current is produced due to strongspin-orbit coupling. As described hereinabove, the produced spin currentmay be used to switch the direction of the magnetization of ananomagnet. By injecting a charge current through a heavy-metal thinfilm, a traverse spin current is produced due to strong spin-orbitcoupling. The produced spin current may be used to switch the directionof the magnetization of a nanomagnet. The magnitude of the produced spincurrent is directly proportional to the spin Hall angle of a thin filmheavy-metal. Large spin Hall angles have been observed in some highresistivity thin films of heavy-metals. It has been shown bothexperimentally and theoretically that the magnitude of the spin Hallangle in some thin film heavy-metals such as, for example, thin films ofW is directly proportional to the resistivity (thickness) of the thinfilm. For example, it has been observed that by increasing the thicknessof a thin film of tungsten from 5.2 nm to 15 nm, the spin Hall angledrops from 0.33 to less than 0.07.

The magnitude of the produced spin current is directly proportional tothe spin Hall angle of a thin film heavy-metal. Large spin Hall angleshave been observed in some high resistivity thin films of heavy-metals.It has been shown both experimentally and theoretically that themagnitude of the measured (calculated) spin Hall angle in thin filmheavy-metals is directly proportional to the resistivity (thickness) ofthe thin film. For example, it has been observed that by increasing thethickness of a thin film of tungsten from 5.2 nm to 15 nm, the spin Hallangle drops from 0.33 to less than 0.07.

In summary with reference to the exemplary embodiment of FIG. 1, a baseelement 100 for switching a magnetization state of a nanomagnet 101includes a heavy-metal strip 102 having a surface 103. The ferromagneticnanomagnet 101 is disposed adjacent to the surface 103 of theheavy-metal strip 102. The ferromagnetic nanomagnet 101 as a firstmagnetization equilibrium state 501 and a second magnetizationequilibrium state 502 (also referred to in some embodiments as an upwardequilibrium state and a downward equilibrium state). The firstmagnetization equilibrium state 501 or the second magnetizationequilibrium state 502 is settable by a flow of electrical charge havingan electrical charge current direction 301 through the heavy-metalstrip. The ferromagnetic nanomagnet can also be a feature of a magneticlayer in an integrated device incorporating the base element.

In some embodiments, by causing a flow of charge (current) in theheavy-metal strip as described hereinabove the magnetization of thenanomagnet can be switched between a first equilibrium state and asecond equilibrium state, such as by reversing the direction of the flowof charge. In some contemplated applications, such as, for example wherethe structure is a base element of a memory or a logic system, the firstequilibrium state can be assigned to either a Boolean “0” or a “1” andthe second equilibrium state can be assigned to the other Boolean numberdifferent from the first equilibrium state. In such contemplatedapplications, the method to change the magnetization as describedhereinabove is analogous to a “write” operation.

Also, in such contemplated applications, methods for reading themagnetization state of a base element are known, such as, for example,by adding an insulating layer over the nanomagnet and another magneticlayer having a fixed magnetization over the insulating layer. When thenanomagnet is switched to a magnetization equilibrium state aboutparallel to the magnetization of the fixed magnetization magnetic layer,there will be a low electrical resistance between the magnetic layerhaving a fixed magnetization and the magnetic layer having a switchablemagnetization. Conversely, when the nanomagnet is switched to amagnetization equilibrium state about anti-parallel to the magnetizationof the fixed magnetization magnetic layer, there will be a highelectrical resistance between the magnetic layer having a fixedmagnetization and the magnetic layer having a switchable magnetization.Thus, in some embodiments, a “read” operation to determine themagnetization state of the base element (e.g. a single “bit”) can beperformed by sensing a low resistance or a high resistance.

It is contemplated that the base element described hereinabove can beused as a bit of an integrated device, such as, for example, a memorydevice or a logic device. In such applications, techniques ofintegration known in the art can be used to form and interconnect aplurality of such base elements. It is contemplated that billions ofsuch base elements with nanomagnets of an integrated magnetic layer canbe integrated into a single integrated device. Internal integratedelectrical connections between base elements can be made usingintegrated circuit interconnection techniques known in the art.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A base element for switching a magnetizationstate of a nanomagnet comprising: a heavy-metal strip having a surface;and a ferromagnetic nanomagnet disposed adjacent to said surface, saidferromagnetic nanomagnet having a first magnetization equilibrium stateand a second magnetization equilibrium state, said first magnetizationequilibrium state or said second magnetization equilibrium statesettable in an absence of an external magnetic field by a flow ofelectrical charge through said heavy-metal strip.
 2. The base element ofclaim 1, wherein said flow of electrical charge in a first directionthrough said heavy-metal strip causes said first magnetizationequilibrium state and said flow of electrical charge in a seconddirection through said heavy-metal strip causes said secondmagnetization equilibrium state.
 3. The base element of claim 1, whereinsaid nanomagnet comprises an elliptical shape having a long axis and ashort axis.
 4. The base element of claim 3, wherein said long axis isabout parallel to said surface of said heavy-metal strip.
 5. The baseelement of claim 4, wherein a direction of flow of said electricalcharge through said heavy-metal strip comprises an angle ξ with respectto said short axis of said nanomagnet.
 6. The base element of claim 5,wherein said angle ξ determines an energy of switching.
 7. The baseelement of claim 5, wherein said angle ξ determines a speed ofswitching.
 8. The base element of claim 1, wherein said base elementprovides a bit of an integrated memory device.
 9. The base element ofclaim 1, wherein said base element provides a bit of an integrated logicdevice.
 10. The base element of claim 1, wherein said base elementprovides a bit of an integrated pipelined microprocessor device.
 11. Thebase element of claim 1, wherein said heavy-metal strip comprisestungsten or tantalum.
 12. The base element of claim 1, wherein saidheavy-metal strip comprises Aluminum (Al) or Gold (Au).
 13. The baseelement of claim 1, wherein said heavy-metal strip comprises Bismuth(Bi) or Molybdenum (Mo).
 14. The base element of claim 1, wherein saidheavy-metal strip comprises Niobium (Nb) or Palladium (Pd).
 15. The baseelement of claim 1, wherein said heavy-metal strip comprises Platinum(Pt).
 16. The base element of claim 1, wherein said heavy-metal stripcomprises an alloy of copper (Cu) and Bi, or an alloy of Cu and iridium(Ir).
 17. A method for switching a magnetization state of a nanomagnetcomprising the steps of: providing a heavy-metal strip having a surfaceand a ferromagnetic nanomagnet disposed adjacent to said surface, saidferromagnetic nanomagnet having a first magnetization equilibrium stateand a second magnetization equilibrium state; and flowing an electricalcharge through said heavy-metal strip in an electrical charge directionto set said magnetization state of said nanomagnet in an absence of anexternal magnetic field to said first magnetization equilibrium state,or to set said magnetization state to said second magnetizationequilibrium state.
 18. The method of claim 17, wherein said step offlowing an electrical charge comprises flowing an electrical chargethrough said heavy-metal strip in an electrical charge direction to setsaid magnetization state within a time period of less than about 50picoseconds.
 19. The method of claim 17, wherein said step of flowing anelectrical charge comprises flowing an electrical charge through saidheavy-metal strip in an electrical charge direction to set saidmagnetization state where said magnetization state corresponds tosetting a bit of a memory device.
 20. The method of claim 17, whereinsaid step of flowing an electrical charge comprises flowing anelectrical charge through said heavy-metal strip in an electrical chargedirection to set said magnetization state where said magnetization statecorresponds to setting a bit of a logic device.